Isodesmic transformations

The peculiar character of this single bond has been demonstrated by calculating the reaction energy for the isodesmic transformation, HCsC—CsCH + 2 C2H2 + C2H5 the... 

The reliability of the above theoretical results obviously depends on the quality of the hypersurface. At the SCF level, only the parts of the surface locally adapted around the stationary points corresponding to equilibrium structures are relatively accurate. For example, the SCF vibrational frequencies are overestimated with respect to experimental data by approximately 10% (Sana, 1981a). SCF heats of reaction and equilibrium constants are generally not satisfactory except for the transformations in which the number of electron pairs is conserved and, particularly, for isodesmic reactions. 

For many strained hydrocarbons and alkenes, heats of formation are not known. In these cases, computational methodology may be used. For a number of molecules, the total energies determined by ab initio calculations and transformed into heats of formation by special group equivalents may be used (81b). Strain energies have also been obtained via isodesmic reactions, which measure deviations from the additivity of bond energies (224). 

---